Terrestrial photovoltaic solar energy, predominantly based on single junction crystalline silicon, has a world record efficiency of 25% which represents an upper limit on the efficiency which we expect commercial modules eventually reach. To go beyond this performance, which is important in order to continuingly drive down the overall cost of generating electricity from sun light, more advanced concepts are required (Shockley et al., J. Appl. Phys. 32, 510 (1961); Yin et al., J. Mater. Chem. A. 3, 8926-8942 (2014) and Polman et al., Nat. Mater. 11, 174-7 (2012)). One such concept is to create a “tandem junction” by employing a wide band gap “top cell” in combination with a silicon “bottom cell”, which could increase the realistically achievable efficiency to beyond 30% (Sivaram et al., Sci. Am. 313, 54-59 (2015)). For maximizing performance, a crystalline silicon (c-Si) bottom-cell with a band gap of 1.1 eV requires a top-cell material with a band gap of ˜1.75 eV, in order to perfectly current-match both junctions (Shah et al., Science. 285, 692-699 (1999)).
However, to date, there has yet to be a suitable wide band gap top cell material for silicon or thin film technologies, which offers stability, high performance, and low cost. In recent years, metal halide perovskite-based solar cells have gained significant attention due to their high power conversion efficiencies (PCE) and low processing cost (C. R. Kagan, Science. 286, 945-947 (1999); Lee et al., Science. 338, 643-7 (2012); Liu et al., Nature. 501, 395-8 (2013); Burschka et al., Nature. 499, 316-9 (2013); Green et al., Nat. Photonics. 8, 506-514 (2014); Jeon et al., Nat. Mater. 13, 1-7 (2014); and Jeon et al., Nature. 517, 476-480 (2015)). An attractive feature of this material is the ability to tune its band gap from 1.48 to 2.3 eV (Noh et al., Nano Lett. 13, 1764-9 (2013) and Eperon et al., Energy Environ. Sci. 7, 982 (2014)) implying that one could potentially fabricate an ideal material for tandem cell applications.
Perovskite-based solar cells are generally fabricated using organic-inorganic trihalide perovskites with the formulation ABX3, where A is the methylammonium (CH3NH3) (MA) or formamidinium (HC(NH2)2) (FA) cation, B is commonly lead (Pb), while X is a halide (Cl, Br, I). Although these perovskite structures offer high power conversion efficiencies (PCE), reaching over 20% PCE with band gaps of around 1.5 eV, fundamental issues have been discovered when attempting to tune their band gaps to hit the optimum 1.7 to 1.8 eV range (Yang et al., Science. 348, 1234-1237 (2015)). In the case of methylammonium lead trihalide (MAPb(I(1-y)Br3)3), Hoke et al. (Chem. Sci. 6, 613-617 (2014)) reported that light-soaking induces a halide segregation within the absorbing material. The formation of iodide-rich domains with lower band gap results in an increase in sub-gap absorption and a red-shift of photoluminescence (PL). The lower band gap regions limit the voltage attainable with such a material, so this band gap “photo-instability” limits the use of MAPb(I(1-y)Br3)3 in tandem devices. In addition, when considering real-world applications, it has been shown that MAPbI3 is inherently thermally unstable at 85° C. even in an inert atmosphere—this is the temperature that international regulations require a commercial PV product to be capable of withstanding.
Concerning the more thermally stable FAPbX3 perovskite, open-circuit voltage (VOC) pinning has also been observed in FAPb(I(1-x)Brx)3 devices, where an increase in optical band gap did not result in an expected increase in VOC. Furthermore, as the iodide is substituted with bromide, a crystal phase transition is observed from a trigonal to a cubic structure: in compositions close to the transition, the material appears unable to crystallize, resulting in an apparently “amorphous” phase with high levels of energetic disorder and unexpectedly low absorption. These compositions additionally have much lower charge-carrier mobilities in the range of 1 cm2/Vs, in comparison to over 20 cm2/Vs in the neat iodide perovskite, and higher recombination rates than in the crystalline material. This is not an issue for high efficiency single junction solar cells since they can be fabricated with the iodine rich, phase stable material, but disadvantageously for tandem applications, this occurs right at the Br composition needed to form the desired top-cell band gap of ˜1.7 to 1.8 eV.
Nevertheless, perovskite/silicon tandem solar cells have already been reported in a 4-terminal and 2-terminal architectures (Bailie et al., Energy Environ. Sci. 8, 956-963 (2015); Löper et al., Phys. Chem. Chem. Phys. 17, 1619-29 (2015); and Mailoa et al., Appl. Phys. Lett. 106, 121105 (2015)). However, their reported efficiencies have yet to surpass the optimized single-junction efficiencies, in part due to non-ideal absorber band gaps having been employed. To avoid the halide segregation problem, it is possible to form a lower band gap triiodide perovskite material and current-match the top and bottom junctions in a monolithic architecture by simply reducing the thickness of the top-cell. However, this method results in a non-ideal efficiency.
Choi et al. (Nano Energy (2014) 7, 80-85) describes mixed cation perovskite compounds comprising both cesium and methylammonium. Lee et al. (Adv. Energy Mater. (2015) 5, 1501310) describes mixed cation perovskite compounds comprising both cesium and formamidinium. Pellet et al. (Angew. Chem. Int. Ed. (2014) 53, 3151-3157) describes mixed cation perovskite compounds comprising both methylammonium and formamidinium. Jeon et al. (Nature (2015) 517, 476-479) describes mixed cation/mixed halide perovskite compounds comprising both methylammonium and formamidinium and both iodide and bromide. However, these perovskite do not yet provide the full tunability and stability required for use in tandem cells.
There is a need to develop a new photoactive material which has a tunable band gap and does not have the issues of halide segregation or thermal instability.